Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.5.4

10.2.5.4

ahorātrāṇi vā upasadaḥ ādityaḥ pravargyo'muṃ tadādityamahorātreṣu pratiṣṭhāpayati tasmādeṣo'horātreṣu pratiṣṭhitaḥ

The Upasads, indeed, are the days and nights, and the Pravargya is the sun: he thus establishes yonder sun on the days and nights, whence he is established on the days and nights.